EFFECTS OF CHEMISORPTION ON THE SURFACE-COMPOSITION OF BIMETALLIC CATALYSTS

We have investigated the effect of hydrogen chemisorption on the surfa ce composition of bimetallic RhPt catalytic clusters using the bond or der metal simulation model. The differences in adsorption energies of hydrogen on Rh and on Pt for one monolayer of hydrogen are obtained by scaling the values for single-hydrogen adsorption, predicted by non-s elf-consistent electron density functional theory, until the results f or surface segregation data from nuclear magnetic resonance are reprod uced, We find the best fit with energy differences of -8.19 and -12.75 kJ/mol for three- and fourfold fee hollow sites, respectively, in exc ellent agreement with the recent calorimetric measurements of -6 kJ/mo l for the average heat of adsorption, In the presence of hydrogen, our cluster surfaces are Rh-rich, especially for the fcc(100) faces, comp letely reversing the segregation of Pt to the surface of the bare clus ter. (C) 1997 Academic Press.